This invention relates to processes for obtaining hydrocarbons from biomass. More particularly, this invention relates to a process for hydroprocessing an acidic biomass feedstock in a guard bed under mild conditions to prevent undesired polymerization from occurring.
A process has recently been developed in which biofeedstocks are deoxygenated by addition of hydrogen to produce a highly-stable green diesel fuel with a higher cetane value, lower cloud point and lower emissions than biodiesel and traditional petrodiesel. This process is feedstock flexible in working with a wide range of biofeedstocks ranging from first generation vegetable oil options to second generation feedstock options such as algal oils and cellulosic feeds. The green diesel fuel that is produced is indistinguishable from traditional diesel fuel and can work as a drop-in replacement or as a valuable blend stock that will enhance the quality of the existing diesel pool. Since it is chemically similar to traditional diesel fuel, green diesel can be used in today's tanks, pipelines, trucks, pumps and automobiles without changes, which will save significant expense as demand for renewables grows.
However, it has been found that some of the biofeedstocks that are used to make green diesel have high levels of free fatty acids (FFA). This high acidity leads the FFAs to polymerize even though they are being hydroconverted under high pressure hydrogen. Unfortunately, this polymerization can result in plugging of the fixed bed reactor resulting in high pressure drop and even unit shutdown.
Another process that has been found to have a need to prevent the polymerization of acids is the hydroprocessing of pyrolysis oil. In a fast thermal process developed by Ensyn Technologies Inc. of Ottawa Canada, a biomass, such as forest residuals or agricultural by-products are rapidly heated to approximately 500° C. in the absence of oxygen. The biomass is vaporized and then rapidly quenched, typically yielding from 65 to 75 wt-% pyrolysis oil. This pourable liquid can then be upgraded to produce transportation fuels. One of the useful processes in making such transportation fuels is hydroprocessing. However, as with the processing of biofeedstocks to make green diesel, pyrolysis oil, due to its high intrinsic acidity, is susceptible to undesired polymerization. Therefore, it has been found necessary to develop a solution to process acidic feeds that polymerize before saturation of the olefins can be accomplished when running at normal hydroconversion conditions.